Catch a Whiff of Galaxy Aromas!

Title: Polycyclic Aromatic Hydrocarbon Emission in Galaxies as seen with JWST

Authors: Dimitra Rigopoulou, Fergus R. Donnan, Ismael García-Bernete, Miguel Pereira-Santaella, Almudena Alonso-Herrero, Ric Davies, Leslie K. Hunt, Patrick F. Roche, Taro Shimizu

First Author’s Institution: Department of Physics, University of Oxford

Status: Accepted to MNRAS [open access]

What Impacts a Galaxy’s Aroma?

Benzene molecule, which contains 6 carbon atoms arranged in a hexagonal ring with each of the carbons having a hydrogen atom attached to them
Figure 1: Chemical structure of benzene (C6H6), which contains 6 carbon atoms in a ring-like structure, with each carbon atom having 1 hydrogen atom attached. The 6 carbons arranged in a ring-like, hexagonal structure at the center create what is called a “benzene ring” (which can have any atoms attached to the sides to form other molecules). PAHs contain at least 2 of these ring-like structures. Image credit: Vladsinger via Wikimedia Commons

Today we’re going to explore how JWST is opening new windows into studying the aromas of galaxies! Unfortunately, these aromas aren’t the type that you’d smell in a candle shop – instead, they refer to the chemical properties of molecules in interstellar dust. We’ll be focused on a particularly abundant type of molecule: polycyclic aromatic hydrocarbons (or PAHs, for short). The name is a bit intimidating, so let’s break it down, moving from back to front. Hydrocarbons are molecules consisting of hydrogen and carbon atoms, strung together with chemical bonds. Methane (CH4) is one example of a common hydrocarbon. An aromatic hydrocarbon is any hydrocarbon that contains a benzene ring, which is a hexagonal compound with 6 carbon atoms arranged in a ring (see Figure 1). Finally, the polycyclic part of the PAH name tells us that there must be more than one benzene ring in our structure.

Astronomers are excited about these complex molecules because they can emit up to ~20% of the total infrared (IR) light from a galaxy. This is because PAHs ring like a bell when they are exposed to more energetic ultraviolet (UV) light; UV light heats up the PAHs causing some of their bonds to vibrate. Vibrations lead to the emission of IR light, and the wavelength of the emission depends on which bonds are vibrating. The amount of vibration in each bond is set by a number of different factors, including the strength of the UV light and both the size and charge of the PAHs. If we understand the interplay between these factors, we can use PAHs to actually get a hold of the physical conditions of galaxies, like the amount of star formation!

Entering the JWST Era

On Christmas Day in 2021, astronomers around the world were given a Christmas present decades in the making – the successful launch of JWST! Now, JWST is bringing us tons of exciting science, including the study of PAHs! The last telescope that could study PAHs was Spitzer, whose spatial resolution was significantly worse than JWST (see Figure 2). This meant that prior to the launch of JWST we could only study PAHs on a global scale (i.e. taking all of a galaxy’s light). The new capabilities of JWST now allow astronomers to study how the local environment (i.e. specific parts of a galaxy) affects the PAHs, something that has been nearly impossible to study until now!

Comparison of JWST MIRI to Spitzer. Image of the same field with each instrument, showing how much better JWST's spatial resolution is compared to previous instruments
Figure 2: Comparison of JWST images (right) to its predecessor (Spitzer, left). JWST has much better angular resolution and depth, which allows for fainter and smaller targets to be observed. Image Credit: NASA/ESA/CSA/STScI

Today’s paper focuses on PAHs in JWST data from 5 nearby galaxies that have a range of physical conditions. Some of these galaxies are starburst galaxies, in which there are a lot of young, hot stars that produce copious amounts of UV radiation to excite the PAHs. Others are what we call active galactic nuclei (AGN), which host supermassive black holes at their centers that are gobbling down gas through a process called accretion. AGN tend to be messy eaters and actually spew out lots of radiation and matter as they accrete; this radiation tends to be more violent than star formation and is thought to destroy many of the bonds that hold the PAHs together. Using these 5 systems with a range of properties, the authors of today’s paper try to use different PAH features to determine what impact the physical properties of the galaxies have on the PAHs.

Introducing: The 17 μm Feature

In their past work, the authors came up with models for how PAH emission in different bands changes as a function of three different factors: (1) the average energy of the UV radiation field, (2) PAH size, and (3) PAH charge (see Figure 3). All three of these play a role in affecting how significantly a PAH molecule will react to the heat deposited by the radiation source. For example, a smaller PAH (with fewer carbon atoms) is easier to heat up and can therefore produce more energetic emission at shorter wavelengths. In today’s paper, they introduce a new feature to these models – a 17 μm PAH feature, which they find is commonly produced by large PAHs (with more carbon atoms). They suggest that the ratio of the 17 μm feature to a 3.3 μm feature (produced by smaller PAHs) can be used to probe the typical size of the PAHs, and that, from this analysis, most sources actually show remarkably similar PAH sizes (see Figure 3)! 

Two ratios of different PAH features. The x-axis has a ratio that scales with the size of the PAHs, whereas the y-axis has a ratio that scales with the charge of the PAHs and the strength of the UV radiation field. Models and data are plotted, highlighting the relatively small range of PAH sizes for various different sources.
Figure 3: Role of the average energy of the UV radiation field, PAH size, and PAH charge on the ratio of different PAH features. The x-axis shows the ratio of the 17 μm feature strength to the 3.3 μm feature strength, which is a function of the average size of the PAHs or equivalently, the average number of carbon atoms per PAH molecule (which is shown in the top color bar). The y-axis shows the ratio of the 11.3 μm feature strength to the 7.7 μm feature strength, which is a function of the charge of the PAHs. The grids on the plot show the authors’ theoretical models, with the top panel showing neutral PAHs and the bottom showing ionized. The strength of the UV radiation field increases as you go further down in the grid. The data points come from JWST data from either the nucleus (i.e. by the supermassive black hole) or star-forming regions. This figure highlights the power of these emission line ratios in telling us about the underlying physics of PAHs (the bell) and the UV radiation field (the mallet that rings it). Modified version of Figure 5 in today’s paper.

What Plays the Biggest Role? 

Next up, the authors investigate some other ratios of PAH features to test which of the three factors mentioned above (UV radiation field, PAH size, and PAH charge) have the biggest impact on the observed PAH emission. Figure 3 shows that both the strength of the UV radiation and the PAH charge impact the ratio of 11.3 to 7.7 μm feature strengths (on the y-axis); it is nearly impossible to decouple neutral PAHs in a strong radiation field from ionized PAHs in a weak radiation field with this ratio alone. So the authors turned to another observable to break this degeneracy – the ratio of two different Neon lines, which directly probes the properties of the UV radiation field. From their analysis, they found that the sources with strong UV radiation fields from Neon line ratios have neutral PAHs of all sizes. Importantly, these findings actually suggest that the AGN are not destroying the smallest PAHs; they’re destroying the ionized PAHs! 

What’s next for PAH studies? Observationally, these findings are good news for studies of PAHs in the distant universe, since the shortest wavelength 3.3 μm PAH feature comes primarily from small, neutral PAHs, which are abundant! Theoretically, there is another key factor on PAH emission that the authors want to explore in future work – the impact of the amount of metals in a galaxy on its PAH features (where, to an astronomer, a metal is any element heavier than hydrogen and helium). Keep your eyes (and nose*) peeled for more exciting results on galaxy aromas!

* While writing this article, this astronomer learned that the term “aromatic” (sadly) has nothing to do with the amount of smell produced by a chemical compound (see the history section of the aromaticity Wikipedia page for more information). 

Astrobite edited by Lucas Brown

Featured image credit: modified from ESA/Webb, NASA & CSA, L. Armus, A. S. Evans

About Megan Masterson

I'm a 4th year PhD student at MIT studying transient accretion events around supermassive black holes, including tidal disruption events and changing-look AGN. I primarily use multi-wavelength observations to study from the inner accretion flow to the obscuring material in these transients. In my free time, you'll find me hiking, reading, and watching women's soccer.

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